Preparation Of Poly(ethylene-Co-Vinyl Alcohol) Membranes By Block Polyether And Their Oil/water Separation Performance

For membrane process in oil/water separation, hydrophilicity polymers are needed for anti-fouling. However, common hydrophilic polymers are readily dissolved or/and swollen in water, consequently lose their mechanical strength. Poly(ethylene-co-vinyl alcohol) (EVAL), consisting of hydrophobic ethylene and hydrophilic vinyl alcohol segments, possesses hydrophilicity owing to the hydroxyl group and at the same time has a high thermal stability, chemical and biological resistance due to the ethylene chain segments.EVAL membranes with different polymer concentration, prepared via immersion precipitation, were used successfully in oil/water separation process. The tensile strength and elongation at break of EVAL membranes increased with an increased polymer concentration while the pure water flux and oil/water emulsion constant flux decreased. However, because of the presence of finger-like macrovoids, the mechanical performances of EVAL membranes are poor even with a high polymer content and the constant flux is still low.In order to improve the oil/water emulsion separation performance of EVAL membrane, F127/EVAL blend membranes were prepared by blending amphiphilic block polyether F127 with EVAL. The mechanical performance and hydrophilicity of F127/EVAL blend membranes were distinctly improved compared with pure EVAL membrane. When the content of F127 increased to 20wt%, the tensile strength, elongation at break, pure water flux and oil/water emulsion constant flux of F127/EVAL blend membrane were 2.55, 1.67, 2.15, 1.66 times, respectively, that of pure EVAL membrane and the flux recovery ratio after washing increased from 48.7% to 82.1%. F127/EVAL blend membranes were reused for the oil/water separation. After a second washing, the flux recovery ratio decreased which is due to the extraction of F127 from the blend membranes in the process of separation and washing.To avoid the losing of F127 in blend membranes, chain extended F127, CEF127, was synthesized by reacting F127 with 3 -Isocyanatomethyl- 3, 5, 5-trimethylcyclohexyl isocyanate (IPDI). When the molar ratio of–NCO and–OH was 2:1, CEF127 with molecular weight twice of F127 was obtained after the reaction. CEF127/EVAL blend membranes were prepared by blending of synthetized CEF127 with EVAL. The mechanical performance of CEF127/EVAL blend membranes improved slightly compared with pure EVAL membrane while the hydrophilicity, pure water flux and oil/water emulsion constant flux were enhanced remarkably. The oil/water emulsion constant flux of CEF127/EVAL blend membrane with 10 wt% CEF127 reached 98.9 L/(m2h) ,which was 3.38 times that of pure EVAL membrane but the rejection ratio declined seriously. And the flux recovery ratio was 68.7 % after first washing, but after the third wash, it dropped to 49 %.In the interest of improving the mechanical performance of EVAL membranes, cross-linked EVAL membranes were prepared by cross-linking EVAL with tetraethyl orthosilicate (TEOS). The mechanical performance of the cross-linked EVAL membranes was improved substantially. The tensile strength and elongation at break of the cross-linked membrane with 1.74 wt% TEOS were 5.11 MPa and 6.72 %, 4.4 and 3.1 times, respectively, that of pure EVAL membrane. When the usages of TEOS were less than 1.74 wt%, the oil/water emulsion constant fluxes of cross linked EVAL membranes were nearly the same as that of pure EVAL membrane while the rejection ratios increased to 97.7 %. When the usages of TEOS were more than 1.74 wt%, the constant fluxes increased but the rejection ratios dropped from 92 % of pure membrane to 73.5 % of membrane with 2.61 wt% TEOS and 49.6 % of membrane with 3.48 wt% TEOS.F127/TEOS/EVAL membranes were prepared by adding both F127 and TEOS in EVAL, in order to improving the mechanical and oil/warer separation performances of EVAL membranes at the same time. But the the tensile strength, elongation at break, oil/water emulsion constant flux, rejection ratio, and flux recovery ratio afer washing of the F127/TEOS/EVAL membranes were were all in the middle of cross linked EVAL membrane and F127/EVAL blend membrane. Which indicated that no synergistic effect existed by F127 and TEOS, and that adding F127 and TEOS at the same time would reduce the performance of membranes only adding one of them.